Every autumn, it’s not just the plants that are “harvesting”…

Produced by: Science Popularization China

Author: Gao Kaixing (Institute of Microbiology, Chinese Academy of Sciences)

Producer: China Science Expo

In autumn, a golden and brilliant season, the land of China is filled with the joy of harvest. The bountiful harvest is the fruit of hard work that year and also the hope for sowing next year.

As the saying goes, "The fragrance of rice flowers heralds a good harvest, and the sound of frogs can be heard." This is true for crops and pathogens. After the abundant fruits are picked, what remains are dead branches and leaves, which deprive a large number of living parasitic pathogens of their nutritional supply and make them unable to survive.

Smart microorganisms also need to ensure that they can reproduce and thrive. For this reason, the autumn harvest season is often also the season when pathogens multiply and wreak havoc.

Pests and diseases: a huge threat to crop growth and harvest

Throughout their life cycle, crops are faced with the threat of pathogenic microorganisms, from the germination of seeds through the soil to the growth of green leaves and the production of fruits.

The roots, leaves and fruits of crops will be infected by different pathogens, resulting in different degrees of damage to the plants. Moreover, when there is abundant rainfall and the temperature and humidity are suitable, pathogens tend to be more rampant. Therefore, the rainy autumn has always been a happy but worrying season for farmers.

All plant organs may be invaded by pathogens

(Image source: Reference [1])

In July 2023, the outbreak of Typhoon Dusurui hit the main grain-producing areas in the Huanghuai and Huaihai regions of Anhui, Henan and other parts of my country, causing massive damage to staple crops such as corn and soybeans, and damaging the leaves and fruits of plants.

In order to prevent the massive outbreak of pests and diseases after disasters, the Ministry of Agriculture and Rural Affairs has specially introduced post-disaster prevention policies to prevent crop yields and reduce economic losses.

It can be seen that the occurrence of diseases and insect pests poses huge potential risks to crop production and harvest.

What diseases can plants get?

So what threats do plants face from pathogens during the autumn harvest season?

Take rice as an example. As the most important staple crop in my country and even in Southeast Asia, the planting area of ​​rice in my country reaches 450 million mu, and the annual output reaches more than 200 million tons. More than 65% of the population in my country rely on rice as their staple food.

The whole growth period of rice is about 4-5 months. During this period, rice goes through a series of links from seeds - small seedlings - tillering - jointing - heading - heading - flowering and finally maturity and fruiting.

During this period, a large number of bacterial, fungal and viral diseases occurred, such as rice white leaf blight, rice stripe leaf blight, rice sheath blight and so on.

Fields infected with rice stripe disease

(Image source: Reference [2])

The most serious rice blast, also known as rice cancer, can occur throughout the entire growing period of rice . Once a large-scale outbreak of rice blast and grain blast occurs during the fruiting period, the entire farmland will be barren.

Rice blast diseased leaves: leaf blast\grain blast

(Image source: Reference [3])

Rice blast is a typical plant fungal disease . Its earliest occurrence can be traced back to a passage in Tiangong Kaiwu written by Song Yingxing in the Ming Dynasty in China: "After the seedlings have sprouted (i.e. heading), ghost fires will burn at night... Whenever the leaves of rice come into contact with them, they will immediately become scorched."

The ancients did not know what rice blast was and compared the diseased plants to being burned by "ghost fire". Now we know that it is actually the spores of the rice blast fungus that are at work.

Rice blast fungus infects plants through the small conidia it produces (shown in the small picture (b) in the figure below). These conidia land on the natural breeze or attach to the bodies of farmers walking through the fields, and then fall onto the leaves of healthy plants.

Rice blast pathogen

(Image source: Reference [4])

These pathogenic microorganisms invade the leaves through wounds caused by the environment or directly through the epidermis, produce attachment cells, form infection filaments, and eventually enter the internal tissues of the plant to complete the infection. Symptoms usually appear within 4-5 days.

If the rice blast fungus infects the grains in the ear when the rice is growing taller and flowering and grafting, it will have a devastating impact on the rice yield.

How should we treat plant diseases?

What is the most effective way to prevent and control plant diseases that develop so quickly and cause such serious harm?

In fact, preventing and controlling plant diseases is just like a doctor treating a human being. First, the doctor will advise the patient to strengthen his immunity through physical exercise. For plants, external fertilizers can be applied to increase plant nutrient absorption , thereby improving their own immunity and enhancing resistance to pathogens.

Once a plant has been infected with pathogens, it needs to be identified to determine whether it is bacteria, fungi or viruses that cause the plant to become "sick", and the appropriate medicines need to be used to treat the disease .

Normally, if it is a bacterial disease, we can make a cross-section of the plant and place the cut surface in water. If a large amount of bacterial pus seeps into the water, it can be determined that the plant has been attacked by bacteria.

Fungal diseases, like rice blast, often produce large numbers of spores, and the presence of fungi can be clearly seen with the naked eye .

Viral diseases are the most difficult to identify because the plant symptoms they cause are similar to those of some non-infectious diseases (plant nutrient deficiency, high temperature stress, etc.). Their identification often requires the use of some molecular biological methods and confirmation under laboratory conditions .

Since plant viruses are usually living parasites, more than 80% of them are transmitted by vector insects. They cleverly take advantage of the feeding process of insect vectors and enter healthy plants along with the insect saliva to complete the infection.

After a virus infects a plant, it often changes the types of compounds it releases from outside the body, reducing the production of insect-resistant substances in the body, thereby helping insects to feed on it. This can be said to be a manifestation of **excellent dual intelligence**.

Therefore, if there are many insects that can spread viruses on some plants suspected of being diseased, it can also help determine the presence of viral diseases.

Insects spread plant diseases

(Image source: author)

Third, treat the disease with the right medicine. For bacterial and fungal diseases, it is often necessary to spray with appropriate pesticides at appropriate dosages, which can effectively slow down the devastating outbreak of the disease and avoid a significant reduction in production. As for viral diseases, there is currently no effective specific medicine to eliminate them. The only way to prevent and control the large-scale outbreak of viral diseases is to control the spread of virus vector insects.

Of course, the use of pesticides often brings a lot of worries to people. Excessive reliance on and use of pesticides poses a great threat to the environment and human health.

Therefore, scientists are constantly exploring and innovating new green methods to prevent and control diseases.

On June 30, 2023, Ye Jian's research group at the Institute of Microbiology, Chinese Academy of Sciences, published their results on disease prevention and control. They found that irradiating plants with near-infrared light (NIR) can enhance their resistance to viral diseases and improve their immune response. They creatively proposed the use of phototherapy for green prevention and control of viral diseases, adding a green magic weapon to the physical toolbox for plant virus prevention and control.

Conclusion

The "love-hate relationship" between plants and microorganisms has continued from ancient times to the present day, and both have been making continuous efforts and evolution for each other's survival.

As humans, we often hope that plants can have the upper hand in the struggle, but don't forget that tiny microorganisms also have extremely high "IQ" and are always ready to take on new challenges.

While we enjoy the joy of harvest, we must not be careless and be careful to prevent diseases from spreading during the autumn harvest.

References:

【1】Agrios, GN, 2005. Plant pathology (Fifth Edition). Academic Press, 3-75. doi:doi.org/10.1016/B978-0-08-047378-9.50007-5. (2005).

【2】Xu, Y., Fu, S., Tao, X., Zhou, X., 2021. Rice stripe virus: Exploring Molecular Weapons in the Arsenal of a Negative-Sense RNA Virus. Annu Rev Phytopathol. doi:10.1146/annurev-phyto-020620-113020 (2021).

【3】Boddy, L., 2016. Pathogens of Autotrophs, The Fungi, pp. 245-292.

【4】Skamnioti, P., Gurr, SJ, 2009. Against the grain: safeguarding rice from rice blast disease. Trends in Biotechnology 27, 141-150. doi:10.1016/j.tibtech.2008.12.002 (2009).

【5】Ye, J., Zhang, L., Zhang, X., Wu, X., Fang, R., 2021. Plant defense networks against insect-borne pathogens. Trends Plant Sci 26, 272-287. doi:10.1016/j.tplants.2020.10.009 (2021).

【6】Zhang,